A timetable fiber Raman ring laser is disclosed in Electronic letters, Vol 26. No. 9. Apr. 26, 1990 page 579-580 which is centered at 1.41 .mu.m and which is pumped at 1.319 .mu.m. Such a construction has a limited output power of 40 mW.
U.S. Pat. No. 5,323,404 describes an Optical Fibre laser or amplifier including high reflectivity gratings. This is a linear design where light is reflected backwards and forwards to stimulate generation of a different wavelength with low efficiency.
A high gain, monolithic, cascade fiber Raman amplifier operating at 1.3 .mu.m is described in Electronic letters, Vol. 31. No. 6. Mar. 16, 1995 pages 472-473. This employs a plurality of loops each loon comprising two optical couplers to complete a closed loop and in order to transfer a Stokes shifted wavelength from the loop. The use of two couplers in each loop is costly and introduces significant losses.
A synchronously pumped ring fiber Raman laser is described in European Patent specification EP-A-0253590 in which a single multiplexing coupler is employed in a reentrant loop to permit stimulated emission at a Stokes shifted wavelength.
This invention has resulted from a consideration of the difficulty of obtaining a high power pump for a ROPA e.g. 1 Watt at 1480 nm. Such a power and wavelength is not currently obtainable by means of semiconductor devices such as previously mentioned. Accordingly, the invention seeks to enable such a drive power to be achieved and has led to the provision of a wavelength converter.